Someone said that photography is the art of painting with light. While most of us aren't a Rembrandt, we need to understand how to apply light in the correct amounts to film, which is our "canvas." With too much light the photo will be white with no detail (over exposed.) With not enough light the photo will be black with no detail (under exposed.) The concepts that we cover this month are extremely important. If anyone has a problem understanding them please email me, I will be happy to help you with it.
The basic measure of light used in photography is the "stop". One stop of film speed, one stop of aperture opening or one stop of shutter speed all equal the same amount of light. A "stop" is like an inch, a cup, or a pound.
It gives us a standard way to measure the amount of something. In photography the "stop" is used to measure exposure. Film speed, aperture opening, shutter speed (and some special filters) are devices used to control exposure. In order to understand photography and to properly expose film YOU MUST HAVE A GOOD GRASP OF THESE.
Film's sensitivity to light is known as ISO or "film speed". The higher the ISO number the faster it will accept light. Each doubling or halving of the ISO number is equal to an increase or decrease of one "stop". 200 ISO film accepts light twice as fast as 100 ISO, 400 ISO is twice as fast 200 ISO, 800 ISO is twice as fast as 400 ISO and so on. Over the next few months we will be talking a lot about film and its relation to light.
Your camera uses two means to control the amount of light that reaches the film. Shutter Speed is the amount of time the shutter remains open and is represented by seconds or fractions of seconds. Each doubling or halving of the shutter speed is equal to an increase or decrease of one "stop" of light. 1/60 is twice as long (twice as much light) as 1/125, 1/250 is twice as long as 1/500 and so on.
Aperture is a bit more difficult to understand because of the mathematical formula used to measure it. The aperture is an opening in the lens which can be made larger or smaller. Light passes through this opening before it reaches the shutter. Actually the aperture setting is also a fraction but is expressed with only the denominator. If you think of f4.0 as 1/4 and f22 as 1/22 you can remember that the higher the aperture number the smaller the opening and therefore the less amount of light that will pass through it.. Aperture settings, usually called "f stops", still follow the same doubling and halving rule as film and shutter speed but in a slightly different way. Instead of doubling or halving at each setting it double or halves every other setting. However, each setting is still one "stop."
Think of it this way. The aperture controls the amount of light and the shutter controls how long that amount of light will be exposed to the film. These two settings share a very close relationship as they determine the proper camera settings for a given exposure value. Both shutter speeds and apertures are sometimes stated in halves or thirds of stops.
Exposure value is simply a value given to the proper exposure of any scene which you meter with your camera. Point your camera toward your subject and (if your using a manual exposure setting) adjust the shutter speed and aperture until the proper exposure is reached. If you're shooting full auto the proper exposure will be automatic. This value is expressed with a combination of shutter speed and aperture opening. While the exposure value remains a constant, different shutter speeds and f stops can be used to achieve it. In order to maintain the proper exposure value of any given scene you must add the same amount of "stops" with shutter speed that you subtract with f stops and vice versa. At a constant ISO film speed 1/125 at f8 is the same as 1/60 at f11 (1/125 to 1/60 is +1 stop, f8 to f11 is -1 stop). If you add or subtract light by changing film speed it must be compensated by either the shutter speed or aperture setting, not both.
If I have a proper exposure of 1/125 of a second at an aperture f8 why would I want to change it? A reasonable question. Shutter speed and aperture can be, and should be, used to obtain a variety of effects. Aperture controls depth of field, flash exposure and some say the sharpness of the picture. Shutter speed can be used to show motion or "freeze" it, as well as to control ambient light. In future articles we will talk more in depth on how to best use shutter speeds and aperture settings to be creative.
Meanwhile, you might want to try these converstions and see if you have an understanding of this most important subject. (Correct answers will appear in my next column.)
At the same Film Speed (ISO), calculate a Shutter Speed:
a) f4 at 1/30 is equivalent to f5.6 at ___?
b) f16 at 1/30 is equivalent to f22 at ____?
c) f16 at 1/125 is equivalent to f8 at ____?
d) f11 at 1/8 is equivalent to f5.6 at ____?
e) f5.6 at 1/250 is equivalent to f22 at ____?